This invention relates to an improved apparatus for nuclear magnetic resonance (NMR) imaging using a Rotating Frame Imaging (RFI) technique, and in particular, to a method and apparatus for NMR tomographic imaging of a sample to provide chemical shift information and micrometer distance resolution for the sample.
RFI employs a gradient in the radio-frequency field (B.sub.1) instead of a gradient in the static main magnetic field (B.sub.0), where B.sub.1 is perpendicular to B.sub.0, to resolve distances associated with respective samples. A major advantage of this method is that the resonance frequency of the nuclear spins associated with the samples is not manipulated to measure the distance. Thus, through the implementation of RFI, associated chemical shifts and distances can be resolved simultaneously.
In the past, it has been difficult to create a B.sub.1 gradient that is both mathematically well defined and strong enough to facilitate resolutions of distances below 0.1 mm using RFI. As a result, RFI was found to be unsuitable for NMR microscopy. Now, however, the subject toroid cavity detector satisfies the requirements for implementation of rotating frame microscopy (RFM) to sample analysis.
When a high pressure environment is needed to conduct sample testing, a pressure vessel is used to contain the toroid cavity monitor and the NMR sample or samples or, in the alternative, the toroid cavity monitor houses the sample which in turn is surrounded by the pressure vessel. The composite unit is placed in an externally applied homogeneous magnetic field (B.sub.0). The presence of B.sub.0 causes the magnetic moments of a targeted class of nuclei in the samples to precess about the field's axis at a rate which is dependent on the magnetic field strength. A coil or toroid internal to the pressure vessel is oriented in such a manner as to apply a magnetic field (B.sub.1) perpendicular to B.sub.0, and then, the B.sub.1 field is alternately energized and de-energized. First, the coil or toroid transmits or energizes, thereby shifting the precession of the magnetic moments away from the B.sub.0 axis, and second, the coil or toroid receives the sample's response as the magnetic moments subsequently return to their equilibrium values. In most applications, the received signal from the energized nuclei serves as the input signal for spectroscopic analysis of the sample.
U.S. Pat. No. 5,045,793 teaches the use of a free-standing, helical wound, toroidal coil as an rf detector for use with a high pressure NMR probe. In the subject invention, the transmitter/receiver coil is replaced by a cylindrical toroid cavity detector. In one embodiment of the present invention, the toroid cavity is a cylindrically shaped, hollow body made of copper with closed ends. A teflon coated wire is fed through the base of the cylinder, and is positioned coincident with the major axis of the cylinder. In an alternate embodiment, slots are machined in the wall, top and bottom of the toroidal cylinder to provide access to the toroid cavity for a fluid or gas sample during the NMR experiment. In a further variation, a series of openings and sample holders positioned in the top of the toroidal cylinder provide access for tubes containing the samples to the toroidal cavity detector or resonator.
Toroidal cavity resonators are known in the prior art, Alderman, however, the subject invention differs from the existing art in the method of tuning and in the structure of the toroid cavity resonator employed. In prior art, the toroid was tuned internally by means of the small gap in the cylindrical axis while the subject invention is externally tuned and no gap exits in the teflon coated wire or central conductor which is oriented along the cylindrical axis. The present invention achieves significant results through the presence of a well-defined magnetic field B.sub.1 gradient, which has been shown theoretically and experimentally to vary with the inverse of the radial displacement from the axial center of the cavity. As a result, the alternating magnetic field is strongest near the central conductor and drops off towards the outer wall of the toroidal cavity detector. The well-defined magnetic field provides a means to image simultaneously both the chemical shift of the targeted nuclei as well as their radial distance from the center.
The amount of energy that is absorbed/transmitted varies with the location of the sample within the toroidal cavity detector because the efficiency of the energy transfer process is proportional to B.sub.1. By varying the transmitter pulse length, multiple distances within the NMR active sample can be resolved. As a result, the major advantage of the new approach is that the chemical shift information is not destroyed by the imaging process as was the case when the measurement was conducted within a homogeneous static magnetic field, B.sub.0, by converting the chemical shift information to a measure of distance with a gradient in B.sub.0. Also, with this invention, the shim system does not need to be altered.
The toroid cavity of the present invention, has several advantages over the prior art in that it provides a rugged reaction chamber that is readily machined from a variety of alloys, and the alternating magnetic field is highly confined within the cavity detector. Confining the alternating magnetic field minimizes sensitivity losses that occur through magnetic coupling with a high-pressure housing. Most importantly, the cavity detector produces a well-defined magnetic field gradient, which, as noted previously, is predicted to vary with the inverse of the radial displacement from the center of the cavity. The resultant NMR intensity (I/I.sub.1) is predicted to depend on the transmitter pulse length, t, according to the following equation EQU I/I.sub.0 =2 .pi.h.intg. sin (.gamma.At/r)dr
where .gamma. is the gyromagnetic ratio, h is the height of the toroid, r is the radial distance from the center of the cavity, and A is the proportionality constant defining the magnetic field as follows: EQU B.sub.1 =A/r
The 1/r relation for the B.sub.1 field suggests that both the NMR sensitivity and the distance resolution should increase for materials that are close to the central conductor. Thus, the toroid cavity NMR resonator is envisioned to be particularly powerful in the characterization of surface layers applied to the central conductor. For example, two layers of tetraflouroethylene fluorocarbon resins were placed around the central conductor of the toroid cavity resonator. The measured thickness of the respective layers was measured as well as their 20 .mu.m separation by means of the NMR imaging data.
Other potential uses for the toroid cavity resonator include in situ monitoring of growth sites during material formation. Additionally, it should be possible to monitor both the gas phase and the active surface sites during chemical vapor deposition by heating the central conductor of the toroid cavity resonator. The toroid cavity detector can be used with a high pressure metal probe housing to conduct measurements at high pressures. Thus, a supercritical fluid at high pressure can be used as the solvent medium to accelerate the diffusion of viscous preceramic polymer reactants into permeable composite materials.
The toroid cavity detectors consist of an inner wire (central conductor) and an outer cylindrical body. With this arrangement, the B.sub.1 field is completely confined within the cavity. It is strongest near the central conductor and drops off as the inverse of the distance toward the outer wall. Both sensitivity and distance resolution increase with the gradient in B.sub.1 and consequently, the toroid detector is well suited for NMR microscopy of films that uniformly surround the central conductor. In addition, the toroid cavity detectors appear to be useful for investigations of solids and polymers where broad lines usually limit the spatial resolution to 50-100 .mu.m. High-pressure and high-temperature capabilities of toroid cavities enable measurements of penetration rates in polymer or ceramic films in situ. In addition, electrochemical processes can be monitored, when the central conductor is used as a working electrode.
Since the B.sub.1 field is strongest near the toroid's central conductor and drops off as the wall is approached, the amount of energy that is absorbed by any sample nuclei present varies with its location within the torus. Differences in the energy absorbed by the nuclei are manifest in their respective pulse rotation angles. Hence, the distances of various types of sample nuclei from the central conductor can be resolved by determining their pulse rotation angles.
Conventional NMR imaging techniques sacrifice all of the spectral details as a side effect of the process used to encode distance information. In contrast, complete NMR spectral information is retained during signal processing with the toroid cavity imaging method. Additionally, because of the strong gradient that is intrinsic to the torus, the theoretical spacial resolution of the toroid cavity imager is better than is possible with conventional MRI. Also, spatial resolution with the toroid cavity imager is less dependent on the line widths of the NMR signals used in the measurements because chemical shift information is not used to determine the distance as it is in conventional MRI.
Rotating Frame Imaging (RFI) uses the B.sub.1 field gradient of an asymmetrically shaped NMR transmitter coil to achieve its spatial resolution. The transmitted energy, and thus, the pulse rotation angle of the NMR active nuclei varies with the B.sub.1 field strength. Accordingly, a gradient can be used for spin localization. Through incremental increases in the pulse width, a set of amplitude-modulated spectra is derived in which the modulation frequencies yield the spatial information. Using a two-dimensional Fourier transformation, both the spatial and the chemical-shift information is resolved. This is a major advantage of the RFI technique over common MRI (Magnetic Resonance Imaging) where the chemical shift information is converted into a measure of distance by using a gradient in the main magnetic field B.sub.0.
NMR microscopy has always suffered from a lack of a B.sub.1 field gradient that is both accurate and strong enough to resolve distances less than 0.1 mm. The toroid cavity detector of the subject invention satisfies RFM (rotating frame microscopy) requirements and facilitates resolution of distances at the micrometer level while simultaneously conserving the chemical-shift information.
Since, the B.sub.1 field of toroid NMR detectors is completely confined within the torus, the use of a cavity instead of a coil to form the torus eliminates the irregularities found in the field of the coil due to the spaces between the individual turns of the coil.
It is the object of this invention to provide a NMR sampling probe which can determine the chemical shift of the targeted nuclei as well as their radial distance from the center.
It is a further object of this invention to teach a toroid cavity detector which is tuned externally.
Another object of this invention provides for sampling the targeted nuclei under high pressure conditions when coupled with a high pressure housing.
Additional advantages, objects and novel features of the invention will become apparent to those skilled in the art upon examination of the following and by practice of the invention.